As a means for detecting a suspicious person intruding into indoor and/or outdoor sites and for giving an alarm, a detecting system using an infrared sensor such as that disclosed in Japanese Patent Laid-Open No. 2001-235367 has been widely used. In the use of the infrared sensor, a light emitting part and a light receiving part are installed at an entrance gate or an opening through which an suspicious person may intrude, and an abnormality can be detected by the light receiving part based on the detection/nondetection of the infrared light sent from the light emitting part, and then, an alarm signal is transmitted.
However, in the detecting system using the infrared sensor, since the alarm signal is transmitted when the light flux of the infrared light is blocked, even in the case where a leaf or the like crosses over the light flux of the infrared light, the detecting system determines that an intruder crosses over it, and resulting in the error detection in many cases. Also, in the case where the detecting system using the infrared sensor is used in the outdoor environment, since the infrared light does not propagate under bad weather conditions such as fog and snow, the detecting system cannot be used in such a condition.
As a method for solving the above-mentioned problems, a detecting system using a radar using a radio wave with microwave bandwidth or millimeterwave bandwidth instead of the infrared sensor has been developed. In the detecting system using an infrared sensor, it is necessary to provide two devices such as a light emitting part and a light receiving part separately. However, in the case of using a radar as a means for detecting an intruder, the detection and monitoring of the intruder can be performed in the state where a transmitting antenna and a receiving antenna are set in a common case box. Therefore, the detecting system using a radar has a high flexibility in installation of the system in comparison to the detecting system using an infrared sensor.
In addition, in the detecting system using an infrared sensor, the simple information whether or not there exists an intruder is all that can be obtained. Meanwhile, in the detecting system using a radar, since it is possible to obtain the surface information of an intruding object, the possibility of the error detection can be extremely reduced. Therefore, the detecting system using a radar can achieve improved performance in comparison to the detecting system using an infrared sensor.
Furthermore, in the detecting system using a radar, the propagation property of the radio wave is little influenced even under bad weather conditions such as fog and snow. Therefore, the detecting system using a radar has improved reliability in comparison to the detecting system using an infrared sensor.
As described above, the detecting system using a radar has various advantages in comparison to the detecting system using an infrared sensor.
However, the size and shape of the area to be monitored are changed depending on the case. In this respect, since only the one-dimensional information whether the infrared light from the light emitting part can be detected at the light receiving part or not can be obtained in the case of the infrared sensor, the installation of the light emitting part and the light receiving part itself determines the area to be monitored. On the other hand, in the case of a radar, a detectable area of the radar is a fan-shaped area principally determined by the maximum detecting distance and the maximum detecting angle which depend on the performance of a transmitting antenna of the radar. Thus, in such a case where the detectable area of the radar is larger than the area to be monitored, the detecting system may detect a person or the like passing outside the monitoring area and give an alarm therefor.